Fellow Spit owner Chris Pugh sent us this pdf issued by the Market Promotion & Training Department. This book is a pocket manual describing specs and rebuild processes of the differential on early Herald, Vitesse, and Spitfires.

Although there are no dates, my guess is they were issued in early 1963.

The file is 2.5 Mb so download will be slow. Also, set your browser to save the file to your hard drive for later reference or Mac: hold down the control button and choose "save link as..." or "download link to disk." Windows: left click (I think) mouse and save to your hard drive.